1. Field of the Invention
This invention relates to an ethylene-vinyl alcohol copolymer (hereinafter referred to as EVOH) composition and a multilayered structure utilizing the same.
2. Description of the Prior Art
EVOH is a useful polymeric material having a high gas barrier property and transparency, as well as oil resistance and odor keeping property, and is widely used for films, sheets, containers and the like.
EVOH is obtained by saponifying ethylene-vinyl acetate copolymer with caustic alkali. The saponified product can, however, not be subjected to melt-molding as it is, since it readily undergoes thermal decomposition which causes a large decrease in viscosity and a severe coloring. A variety of means have been employed to improve the above. For example, Examined Published Japanese Patent Application Nos. 37664/1971 and 19242/1980 and Unexamined Japanese Patent Laid-Open Nos. 25048/1973, 88544/1976, 88545/1976 disclose the improvements by fully washing EVOH, adding an acid to EVOH, and by dipping EVOH in an acid solution. Further, Unexamined Japanese Patent Laid-Open Nos. 954/1977, 955/1977 and 41204/1981 disclose a method which comprises improving the melt-moldability by adding a metal salt on the ground that metal salts of some type have marked effects of thermal stabilization.
The above-mentioned processes all principally intend to decrease a time-dependent viscosity change in the melting operation, and can certainly give molded articles having good appearances when a short-period melt-molding is conducted. However, at a continuous melt molding over 24 hours or longer, galled matters or streaky irregularities generate on the molded article, causing the obtained articles to have bad appearances and finally to be unusable.
The above-described phenomena take place also when co-extruding or co-injecting an EVOH with a hydrophobic thermoplastic resin. Examined Published Japanese Patent Application Nos. 2984/1985, 35125/1983 and 39548/1985 disclose a method which comprises co-extruding an EVOH with a hydrophobic thermoplastic resin and, interposed between the two, another thermoplastic resin modified with an ethylenically unsaturated carboxylic acid or its anhydride. The process however has a drawback of the obtained multilayered structure gradually getting poor in its appearance.